Sign up to receive free email alerts when patent applications with chosen keywords are publishedSIGN UP

Abstract:

Preparing a flexographic member (60) includes providing a digital image
and calculating a relief image based on the digital image. At least one
stress-sensitive boundary region (11) adjacent to at least one image
feature is identified and the relief image is created on the flexographic
member. The depth (18) of at least a portion of a floor region (10)
adjacent the at least one image feature is increased to provide a
modified floor region.

Claims:

1. An apparatus for preparing a flexographic plate comprising: a digital
image for printing on the flexographic plate; calculating a relief image
based on the digital image; identifying at least one boundary region
adjacent to at least one image feature; a laser for printing the digital
image on the flexographic plate; and increasing a depth of at least a
portion of a floor region adjacent the at least one image feature with
the laser.

2. The apparatus of claim 1 wherein a minimum width of the increase in
the floor region is at least 2 μm.

3. The apparatus of claim 1 wherein a maximum width of the increase in
the floor region is a distance to a top of the next image feature.

4. The apparatus of claim 1 wherein a minimum depth of the increase in
the floor region is at least 2 μm.

5. The apparatus of claim 1 wherein a maximum depth of the increase in
the floor region is less than a depth of a backing layer of the
flexographic plate.

6. The apparatus of claim 1 wherein the depth of the floor region is
increased by laser engraving.

7. The apparatus of claim 1 wherein the digital image is stored on a
memory device.

8. The apparatus of claim 1 wherein relief image is calculated on a
computer.

[0002] The present invention relates in general to flexographic printing
and in particular to providing floor relief for dot improvement.

BACKGROUND OF THE INVENTION

[0003] Flexography is a method of printing that is commonly used for
high-volume relief printing runs on a variety of substrates such as
paper, paper stock board, corrugated board, polymeric films, labels,
foils, fabrics, and laminates. Flexographic printing has found particular
application in packaging, where it has displaced rotogravure and offset
lithography printing techniques in many cases.

[0004] Flexographic printing members are sometimes known as "relief
printing members" and are provided with raised relief images onto which
ink is applied for application to a receiver element of some type. The
raised relief images are inked in contrast to the relief "floor" that
remains free of ink. Such flexographic printing members (such as
flexographic printing plates) are supplied to the user as an article
having one or more layers optionally on a substrate or backing material.
Flexographic printing can be carried out using flexographic printing
plates as well as flexographic printing cylinders or seamless sleeves
having a desired relief image.

[0005] Generally, flexographic printing members are produced from a
photosensitive resin or elastomeric rubber. A photo-mask, bearing an
image pattern can be placed over the photosensitive resin sheet and the
resulting masked resin is exposed to light, typically UV radiation, to
crosslink the exposed portions of the resin, followed by developing
treatment in which the unexposed portions (non-crosslinked) of the resin
are washed away with a developing liquid. Recent developments have
introduced the CTP (computer-to-plate) method of creating the mask for
the photosensitive resin. In this method, a thin (generally 1-5 μm in
thickness) light absorption black layer is formed on the surface of the
photosensitive resin plate and the resulting printing plate is irradiated
imagewise with an infrared laser to ablate portions of the mask on the
resin plate directly without separately preparing the mask. In such
systems, only the mask is ablated without ablating the photosensitive
plate precursor. Subsequently, the photosensitive plate precursor is
imagewise exposed to UV light through the ablated areas of the mask, to
crosslink (or harden) the exposed portions of the photosensitive resin,
followed by developing treatment in which the unexposed portions
(uncrosslinked) of the resin and the remaining black mask layer are
washed away with a developing liquid. Both these methods involve a
developing treatment that requires the use of large quantities of liquids
and solvents that subsequently need to be disposed of In addition, the
efficiency in producing flexographic printing plates is limited by the
additional drying time of the developed plates that is required to remove
the developing liquid and dry the plate. Often additional steps of
post-UV exposure or other treatments are needed to harden the surface of
the imaged printing plate.

[0006] While the quality of articles printed using flexographic printing
members has improved significantly as the technology has matured,
physical limitations related to the process of creating a relief image in
a printing member still remain.

[0007] In the flexographic printing process, a flexographic printing
member having a three-dimensional relief image formed in the printing
surface is pressed against an inking unit (normally an Anilox roller) in
order to provide ink on the topmost surface of the relief image. The
inked raised areas are subsequently pressed against a suitable substrate
that is mounted on an impression cylinder. As the flexographic printing
member and Anilox or substrate are adjusted or limited mechanically, the
height of the topmost surface determines the amount of physical
impression pressure between the flexographic printing member and the
Anilox or the flexographic printing member and the substrate. Areas in
the relief image that are raised higher than others will produce more
impression than those that are lower or even recessed. Therefore, the
flexographic printing process is highly sensitive to the impression
pressure that may affect the resulting image. Thus, the impression
pressure must be carefully controlled. If the impression pressure is too
high, some image areas can be squeezed and distorted, and if it is too
low, ink transfer is insufficient. To provide the desired images, a
pressman may test impression pressure settings for a given flexographic
printing plate.

[0008] In particular, it is very difficult to print graphic images with
fine dots, lines, and even text using flexographic printing members. In
the lightest areas of the image (commonly referred to as "highlights"),
the density of the image is represented by the total area of printed dots
in a halftone screen representation of a continuous tone image. For
amplitude modulated (AM) screening, this involves shrinking a plurality
of halftone dots located on a fixed periodic grid to a very small size,
the density of the highlight being represented by the area of the
halftone dots. For frequency modulated (FM) screening, the size of the
halftone dots is generally maintained at some fixed value, and the number
of randomly or pseudo-randomly placed halftone dots represent the density
of the image. In both of these situations, it is necessary to print very
small dot sizes to adequately represent the highlight areas.

[0009] Maintaining small halftone dots on a flexographic printing member
is very difficult due to the nature of the plate making process and the
small size and lack of stability in the halftone dots. Digital
flexographic printing precursors usually have an integral UV-opaque mask
layer coated over a photopolymer or photosensitive layer in the relief
image. In a pre-imaging (or post-imaging) step, the floor of the relief
image in the printing member is set by area exposure to UV light from the
back of the printing precursor. This exposure hardens the photopolymer to
the relief depth required for optimal printing. This step is followed by
selective ablation of the mask layer with an imagewise addressable high
power laser to form an image mask that is opaque to ultraviolet (UV)
light in non-ablated areas. Flood exposure to image-forming UV radiation
and chemical processing are then carried out so that the areas not
exposed to UV are removed in a processing apparatus using developing
solvents, or by a heating and wicking process. The combination of the
mask and UV exposure produces relief halftone dots that have a generally
conical shape. The smallest of these halftone dots are prone to being
removed during processing, which means no ink is transferred to these
areas during printing (the halftone dot is not "held" or formed on the
printing plate or on the printing press). Alternatively, if the small
halftone dots survive processing, they are susceptible to damage on
press. For example, small halftone dots often fold over or partially
break off during printing, causing either excess ink, or no ink, to be
transferred.

[0010] Conventional preparation of non-digital flexographic printing
plates follows a similar process except that the integral mask is
replaced by a separate film mask or "photo-tool" that is imaged
separately and placed in contact with the flexographic printing precursor
under a vacuum frame for the image-forming UV exposure.

[0011] A solution to overcome the highlight problem noted above is to
establish a minimum halftone dot size during printing. This minimum
halftone dot size must be large enough to survive processing, and be able
to withstand printing pressure. Once this ideal halftone dot size is
determined, a "bump" curve can be created that increases the size of the
lower halftone dot values to the minimum halftone dot setting. However,
this results in a loss of the dynamic range and detail in the highlight
and shadow areas. Overall, there is less tonality and detail in the
image.

[0012] Thus, it is well known that there is a limit to the minimum size of
halftone dots that can be reliably represented on a flexographic printing
member and subsequently printed onto a receiver element. The actual
minimum size will vary with a variety of factors including printing
flexographic printing member type, ink used for printing, and imaging
device characteristics among other factors including the particular
printing press that is used. This creates a problem in the highlight
areas when using conventional AM screening since once the minimum
halftone dot size is reached, further size reductions will generally have
unpredictable results. If, for example, the minimum size halftone dot
that can be printed is a 50×50 μm square dot, corresponding to a
5% tone at 114 lines per inch screen frequency, then it becomes very
difficult to faithfully reproduce tones between 0% and 5%. A common
design around this problem is to increase the highlight values in the
original file to ensure that after imaging and processing, all the tonal
values in the file are reproduced as printing dots and are properly
formed on the printing member. However, a disadvantage of this practice
is the resulting additional dot gain in the highlights that causes a
noticeable transition between inked and non-inked areas.

[0013] Another known practical way of improving highlights is through the
use of "Respi" or "double dot" screening as discussed in U.S. Pat. No.
7,486,420 (McCrea et al.). The problem with this type of screening
technique, when applied to flexographic printing, is that the size of
halftone dot that may be printed in isolation is actually quite large,
typically 40-50 μm in diameter. Even when using this technique, the
highlights are difficult to reproduce without having a grainy appearance,
which occurs when halftone dots are spaced far apart to represent a very
low density, and the printed halftone dot may also suffer an undesirable
dot gain.

[0014] U.S. Pat. No. 7,486,420 discloses a flexographic screening
technique that compensates for characteristic printing problems in
highlight areas by selectively placing non-printing dots or pixels
proximate to highlight dots. The non-printing dots or pixels raise the
printing relief floor in the highlight areas providing additional support
for marginally printable image features. This technique allows an image
feature to be surrounded by one or more smaller non-printing features to
provide an extra base of support for the image feature. While this
provides an important advance in the art, it may not always completely
eliminate the grainy appearance in the image.

[0015] MAXTONE screening (Eastman Kodak Company) is a known hybrid AM
screening solution that overcomes some highlight and shadow reproduction
limitations. MAXTONE screening software allows the operator to set a
minimum dot size in order to prevent the formation of halftone dots that
are too small for the flexographic medium. To extend the tonal range,
MAXTONE screening software uses an FM-like screening technique in the
highlights and shadows. To create lighter shades, dots are removed in a
random pattern. By producing lighter colors with fewer (rather than
smaller) halftone dots, improved highlight detail and a more robust
flexographic printing plate are achieved. However, completely removing
dots from a highlight will necessarily reduce the resolution and edge
fidelity of the resulting printed images.

[0016] U.S. Pat. Nos. 5,892,588 and 6,445,465 (both Samworth) describe an
apparatus and method for producing a halftone screen having a plurality
of halftone dots arrayed along a desired screen frequency by deleting a
number of halftone dots per unit area to obtain gray shades below a
predetermined shade of gray.

[0017] Part of the problem of reproducing highlight dots, particularly
when the relief pattern is formed by laser engraving, arise from the
phenomenon of undercutting, or "natural" undercutting, where the top most
surfaces of the smallest features are formed well below the top most
surface of the flexographic printing plate due to details of the laser
engraving process. This is distinct from "intentional" undercutting where
laser intensity is used to purposefully reduce the level of the top most
surface of a relief image feature. The terms "natural" or "naturally"
imply unavoidable undercutting and is system dependent in that as the
laser spot size and resolution of the engraving engine improves the size
of features "naturally" undercut will be smaller.

[0018] Direct engraved printing members can typically suffer loss of
highlights due to undercutting. A Feb. 1, 2010 publication by the
Association of Japanese Flexo Printing Industry entitled "Direct Laser
Plate Making Consideration for Current Status" describes the use of
undercutting in preparing flexographic printing plates to release the
printing pressure in the highlight areas. FIG. 7 in that publication
shows a progressive undercutting in the relief image as the feature size
is reduced. If undercutting is small, the relief in pressure on press may
be desirable but when the undercutting is too great print quality
suffers.

[0019] U.S Publication No. 2009/0223397 (Miyagawa et al.) describes an
apparatus for forming a direct engraved convex dot on a flexographic
printing plate using a light power of the light beam, which engraves all
or part of an adjacent region which is adjacent to a convex portion which
is to be left in a convex shape on a surface of the recording medium, is
equal to or less than a threshold engraving energy, and at a region in
the vicinity of an outer side of the adjacent region, the light power of
the light beam is increased to a level higher than the light power used
in the adjacent region. This may help to reduce the severity of
undercutting by limiting the exposure at the top of the feature but will
not eliminate the problem for the finest engraved features desirable.

[0020] Commonly-assigned copending U.S. patent application Ser. No.
12/868,039 proposed addressing this problem by using a combination of AM,
FM, and engagement modulation, EM, screening where in a sub-area has dots
each having a minimum receiver element contact area, and wherein a
fraction of the dots has a topmost surface that is below the elastomeric
topmost surface, but above the level that will transfer ink on press.
This method can create a smoother tone scale but may be sensitive to
variation of engagement for different press conditions.

[0021] In addition to these problems there are a number of inter-image
effects that result from the proximity of highlight dots and other fine
features that are "naturally" undercut to other image features such as
solids, lines, and text. For example, in a field of highlight dots
adjacent to a solid or a line or surrounded by lines, the row or rows of
dots immediately proximate to the neighboring feature will lose density
on the printed receiver or fail to print entirely resulting in
undesirable non-uniformities.

[0022] Another inter-image effect can be observed when thin lines are
proximate to solids, text or similar features. In that case a line
intended to be straight will appear distorted near the neighboring
feature. The line can appear curved, thicker or thinner.

[0023] Inter-image defects also occur when a field of highlight dots is
adjacent to an extended area of background (i.e. an area of no printable
features). In this case the dots in the last, or last few rows adjacent
to the boundary often print darker than the average dots in the field and
exhibit more halo effect.

[0024] Despite all of the progress made in flexographic printing to
improve image quality in the highlight areas, there remains a need to
improve the representation of small halftone dots and thin lines in
printed flexographic images so that image detail is improved and dot gain
is reduced.

SUMMARY OF THE INVENTION

[0025] Briefly, according to one aspect of the present invention,
preparing a flexographic member includes providing a digital image and
calculating a relief image based on the digital image. At least one
stress-sensitive boundary region adjacent to at least one image feature
is identified and the relief image is created on the flexographic member.
The depth of at least a portion of a floor region adjacent the at least
one image feature is increased to provide a modified floor region

[0026] According to another aspect of the present invention a method of
preparing a flexographic printing member includes the steps of forming a
relief image that consists of at least coarse-featured regions but can
also include fine-featured regions. A cut of increased relief is added at
boundaries according to a procedure determined by the image content.

[0027] The present invention provides a method of preparing a flexographic
printing member used to transfer ink from an image area to a receiver
element, the flexographic printing member comprising a relief image
having an image area composed of an elastomeric composition that has an
elastomeric topmost surface, and a relief image floor. The method
includes the steps of forming a relief image by means of direct laser
engraving and an additional step of engraving cuts of increased relief
image-wise. The step of adding these cuts can occur before, during or
after the formation of the relief pattern. The cuts are intended to
ameliorate the inter-image defects that occur due to undesirable
transmission of stresses in the flexographic member on press during the
printing operation. Cuts of increased relief can achieve these objectives
without greatly increasing the burdens of exposure energy, material
collection and material disposal associated with increasing the entire
floor relief of the flexographic member.

[0028] The invention and its objects and advantages will become more
apparent in the detailed description of the preferred embodiment
presented below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] FIG. 1a is a schematic cross-sectional diagram illustrating a
comparative flexographic member or sleeve having coarse features and a
floor.

[0030] FIG. 1b is schematic cross-sectional diagram of an embodiment of
the current invention showing a cut of increased relief adjacent to at
least one image feature.

[0031] FIG. 2a is a schematic cross-sectional diagram illustrating a
comparative flexographic member or sleeve showing a sloped wall adjacent
the at least one coarse feature region.

[0032] FIG. 2b is schematic cross-sectional diagram of a flexographic
member or sleeve showing of the current invention having a sloped wall
adjacent to at least one coarse feature region and a cut of increased
relief adjacent to said coarse feature region.

[0033] FIG. 3a is a schematic cross-sectional diagram illustrating a
comparative flexographic member or sleeve having fine features adjacent
to coarse features.

[0034] FIG. 3b is schematic cross-sectional diagram of the current
invention showing cuts of increased relief between coarse and fine
features.

[0035] FIG. 4 is a schematic diagram of a laser engraving apparatus used
to implement the steps of the current invention.

DETAILED DESCRIPTION OF THE INVENTION

[0036] The present invention will be directed in particular to elements
forming part of, or in cooperation more directly with the apparatus in
accordance with the present invention. It is to be understood that
elements not specifically shown or described may take various forms well
known to those skilled in the art.

Definitions

[0037] The following definitions identify various terms and phrases used
in this disclosure to define the present invention. Unless otherwise
noted, these definitions are meant to exclude other definitions of the
terms or phrases that may be found in the prior art.

[0038] The term "flexographic printing precursor" refers to the material
that is used to prepare the flexographic printing member of this
invention and can be in the form of flexographic printing plate
precursors, flexographic printing cylinder precursors, and flexographic
printing sleeve precursors.

[0039] The term "flexographic printing member" or "flexographic member"
refers to articles of the present invention that are imaged flexographic
printing precursors and can be in the form of a printing plate having a
substantially planar elastomeric topmost surface, or a printing cylinder
or seamless printing sleeve having a curved elastomeric topmost surface.
In the case of sleeves and cylinders heights and levels are, of course,
in reference to the radial direction.

[0040] The term "receiver element" refers to any material or substrate
that can be printed with ink using a flexographic printing member of this
invention.

[0041] The term "ablative" relates to a composition or layer that can be
imaged using a radiation source (such as a laser) that produces heat
within the layer that causes rapid local changes in the composition or
layer so that the imaged regions are physically detached from the rest of
the composition or layer and ejected from the composition or layer.

[0042] "Ablation imaging" is also known as "ablation engraving", "laser
engraving" or "direct engraving".

[0043] The "elastomeric topmost surface" refers to the outermost surface
of the elastomeric composition or layer in which a relief image is formed
and is the first surface that is struck by imaging radiation.

[0044] The term "relief image" refers to all of the topographical features
of the flexographic printing member provided by imaging and designed to
transfer a pattern of ink to a receiver element.

[0045] The term "image area" refers to a predetermined area of the relief
image in the elastomeric composition, which predetermined area is
designed to be inked and to provide a corresponding inked image area on a
receiver element.

[0046] The term "relief image floor" or "floor" refers to the bottom-most
surface of the relief image excluding any cuts of increased relief as
specified by the current invention. For example, the floor can be
considered the maximum depth of the relief image from the elastomeric
topmost surface and can typically range from 100 to 1000 μm. The
relief image generally includes "valleys" that are not inked and that
have a depth from the elastomeric topmost surface that is less than the
maximum depth.

[0047] As used herein, the term "dot" refers to a formed protrusion or
microstructure in the relief image formed in the flexographic printing
member of this invention. Some publications refer to this dot as a
"halftone dot". The term "dot" does not refer to the dot-like printed
image on a receiver element that is provided by the dot on the
flexographic printing member. However, it is desired that the dot surface
area on the flexographic printing member would correspond as closely as
possible to the dot-like image printed on a receiver element. Dots in the
relief image smaller than a minimum dot size usually determined by
specifics of the laser beam and print engine used to produce it are
typically formed with top most surfaces that are below the original
un-engraved surface of the member. This condition is referred to as
undercutting or "natural" undercutting. A current estimate for the
minimum dot size, given the best engraving systems currently available,
would be approximately 30 μm by 30 μm or 900 μm2 but
smaller features that do not suffer from natural undercutting could
become feasible as system resolution improves.

[0048] The term "fine feature" refers to any relief image feature intended
to transfer ink to a receiver that is "naturally" undercut including such
features as half-tone dots, stand-alone dots, fine lines, small point
text or any other feature having its top most surface about 30 microns or
more below the origin top most surface of the pre-engraved flexographic
printing member due to the limitations of the engraving engine used to
produce the relief image. A fine feature region is defined as any
contiguous area of the engraved flexographic member containing only fine
features.

[0049] The term "coarse feature" refers to any relief image feature
intended to transfer ink to a receiver that can be formed with it top
most surface within about 30 microns of the original top most surface of
the pre-engraved flexographic printing member. A coarse feature region is
defined as any contiguous area of the engraved flexographic member
containing only coarse features. Thus all features intended to transfer
ink to a receiver are either "coarse" or "fine" features and all of the
image area of the flexographic printing member can be subdivide into
"coarse" and "fine" regions. Both coarse-feature regions and fine
featured regions can contain contiguous floor-regions having no printable
features. The top most surface of a floor region is below the level that
will transfer ink on a flexographic printing press under normal printing
conditions.

[0050] Fine-featured relief is defined as any relief feature that is
"naturally" undercut, including such features as half-tone dots,
stand-alone dots, fine lines, small point text or any other feature.
Naturally undercut means that the top most surface of the fine features
is 30 microns or more below the origin top most surface of the
pre-engraved flexographic printing member due to the limitations of the
direct engraving engine used to produce the relief image. These are the
features that cannot be formed with a given engraving engine without
having their top most surface undercut 30 microns or more below the
original surface of the flexographic printing member. With the current
state of technology these fine-features typically have a shortest lateral
linear dimension of about 30 microns or less. One objective of the
current invention is intended to circumvent or ameliorate the deleterious
effects that occur in flexographic printing on press due to natural
undercutting. A fine-feature region is defined as any contiguous area of
the engraved flexographic member containing only fine features.

[0051] In contrast, coarse features are those having lateral linear
dimensions large enough to ensure that the top most surface of the imaged
feature can be left substantially undisturbed by the engraving process
when no additional leveling procedure is employed. These features are
commonly solids, mid range half-tone dots and shoulder half-tone dots,
wide lines and larger point text typically having a shortest lateral
linear dimension on the order of 30 microns or more. A coarse feature
region is defined as any contiguous area of the engraved flexographic
member containing only coarse features.

[0052] Relief features are typically engraved into the flexographic
printing member by scanning a single spot or multiple laser spots of
intense, modulated and focused radiation over the surface of the member
in the image area and collecting the ablated debris. The laser spots can
be scanned over the image area of the member once or several times to
control the depth of ablation. Each scan is commonly referred to as a
pass. During each pass all, or part, of the image relief pattern can be
addressed with predetermined laser intensity image-wise to affect the
depth of ablation at every position in the final relief image.

[0053] Boundaries that benefit from cuts typically occur between coarse
featured regions and floor-regions or between coarse-featured regions and
fine-featured regions. These boundaries occur where stresses on press are
transmitted through the flexographic member causing distortions resulting
in objectionable inter-image effects in the print. These interfaces are
referred to as stress-sensitive boundaries. There are several types of
stress-sensitive boundaries. When, for example, a field of highlight or
mid-tone dots is adjacent to an extended floor region the outer row or
rows of dots print more heavily and exhibit more halo effect than the
dots away from the boundary. This is called a floor-interface
stress-sensitive boundary. The extent of the floor region (or floor
length) refers to the shortest distance between a stress-sensitive
boundary and the next nearest printable feature and determines the
severity of the defect. Modeling and press data indicate that extended
floor regions as short as 2 mm can be problematic for stress-sensitive
boundaries and the problem becomes more severe as the distance increases.
"Cuts of increased relief' or "cuts" refer to engraving increased depth
of a portion of a floor region or inter-dot relief at a boundary adjacent
to at least one image feature at a stress-sensitive boundary.

[0054] Another type, referred to as a fine coarse interface
stress-sensitive boundary, occurs between coarse feature regions and fine
feature regions. In this case modeling and press data indicate that the
stress-sensitive boundary causes the last row or rows of fine features
closest to the interface to print less densely or not at all as compared
with the fine features farther from the boundary.

[0055] Other types of stress-sensitive boundaries can occur, leading to
inter-image artifacts in prints. For example thin lines near text can
appear wavy or narrowed, indicating a stress sensitive boundary between
these features. Inter-image effects can appear as density
non-uniformities or feature placement errors. Cuts at these
stress-sensitive boundaries can reduce or eliminate the problem.

Flexographic Printing Members

[0056] The flexographic printing members prepared using the present
invention can be flexographic printing plates having any suitable shape,
flexographic printing cylinders, or seamless sleeves that are slipped
onto printing cylinders.

[0057] Elastomeric compositions used to prepare useful flexographic
printing precursors are described in numerous publications including, but
not limited to, U.S. Pat. No. 5,719,009 (Fan); U.S. Pat. No. 5,798,202
(Cushner et al.); U.S. Pat. No. 5,804,353 (Cushner et al.); and WO
2005/084959 (Figov), all of which are incorporated herein by reference
with respect to their teaching of laser-ablatable materials and
construction of flexographic printing precursors. In general, the
elastomeric composition comprises a crosslinked elastomer or a vulcanized
rubber.

[0058] DuPont's Cyrel® FAST® thermal mass transfer plates are
commercially available photosensitive resin flexographic printing plate
precursors that comprise an integrated ablatable mask element and require
minimal chemical processing. These elements can be used as flexographic
printing precursors in the practice of this invention.

[0059] For example, flexographic printing precursors can include a
self-supporting laser-ablatable or engraveable, relief-forming layer
(defined below) containing an elastomeric composition that forms a rubber
or elastomeric layer. This layer does not need a separate substrate to
have physical integrity and strength. In such embodiments, the
laser-ablatable, relief-forming layer composed of the elastomeric
composition is thick enough and laser ablation is controlled in such a
manner that the relief image depth is less than the entire thickness, for
example up to 80% of the entire thickness of the layer.

[0060] However, in other embodiments, the flexographic printing precursors
include a suitable dimensionally stable, non-laser engraveable substrate
having an imaging side and a non-imaging side. The substrate has at least
one laser engraveable, relief-forming layer (formed of the elastomeric
composition) disposed on the imaging side. Suitable substrates include
but are not limited to, dimensionally stable polymeric films, aluminum
sheets or cylinders, transparent foams, ceramics, fabrics, or laminates
of polymeric films (from condensation or addition polymers) and metal
sheets such as a laminate of a polyester and aluminum sheet or
polyester/polyamide laminates, or a laminate of a polyester film and a
compliant or adhesive support. Polyester, polycarbonate, vinyl polymer,
and polystyrene films are typically used. Useful polyesters include but
are not limited to poly(ethylene terephthalate) and poly(ethylene
naphthalate). The substrates can have any suitable thickness, but
generally they are at least 0.01 mm or more preferably from about 0.05 to
about 0.3 mm thick, especially for the polymeric substrates. An adhesive
layer may be used to secure the elastomeric composition to the substrate.

[0061] There may be a non-laser ablatable backcoat on the non-imaging side
of the substrate (if present) that may be composed of a soft rubber or
foam, or other compliant layer. This backcoat may be present to provide
adhesion between the substrate and the printing press rollers and to
provide extra compliance to the resulting printing member, or to reduce
or control the curl of the printing member.

[0062] Thus, the flexographic printing precursor contains one or more
layers. Besides the laser-engraveable, relief-forming layer, there may be
a non-laser ablatable elastomeric rubber layer (for example, a cushioning
layer) between the substrate and the topmost elastomeric composition
forming the laser-engraveable relief-forming layer.

[0063] In general, the laser-engraveable, relief-forming layer composed of
the elastomeric composition has a thickness of at least 50 μm and
preferably from about 50 to about 4,000 μm, or more preferably from
200 to 2,000 μm.

[0064] The elastomeric composition includes one or more laser-ablatable
polymeric binders such as crosslinked elastomers or rubbery resins such
as vulcanized rubbers. For example, the elastomeric composition can
include one or more thermosetting or thermoplastic urethane resins that
are derived from the reaction of a polyol (such as polymeric diol or
triol) with a polyisocyanate, or the reaction of a polyamine with a
polyisocyanate. In other embodiments, the elastomeric composition
contains a thermoplastic elastomer and a thermally initiated reaction
product of a multifunctional monomer or oligomer.

[0065] Other elastomeric resins include copolymers or styrene and
butadiene, copolymers of isoprene and styrene, styrene-butadiene-styrene
block copolymers, styrene-isoprene-styrene copolymers, other
polybutadiene or polyisoprene elastomers, nitrile elastomers,
polychloroprene, polyisobutylene and other butyl elastomers, any
elastomers containing chlorosulfonated polyethylene, polysulfide,
polyalkylene oxides, or polyphosphazenes, elastomeric polymers of
(meth)acrylates, elastomeric polyesters, and other similar polymers known
in the art.

[0067] Still other useful laser-engraveable resins are polymeric materials
that, upon heating to 300° C. (generally under nitrogen) at a rate
of 10° C./minute, lose at least 60% (typically at least 90%) of
their mass and form identifiable low molecular weight products that
usually have a molecular weight of 200 or less. Specific examples of such
laser engraveable materials include but are not limited to,
poly(cyanoacrylate)s that include recurring units derived from at least
one alkyl-2-cyanoacrylate monomer and that forms such monomer as the
predominant low molecular weight product during ablation. These polymers
can be homopolymers of a single cyanoacrylate monomer or copolymers
derived from one or more different cyanoacrylate monomers, and optionally
other ethylenically unsaturated polymerizable monomers such as
(meth)acrylate, (meth)acrylamides, vinyl ethers, butadienes,
(meth)acrylic acid, vinyl pyridine, vinyl phosphonic acid, vinyl sulfonic
acid, and styrene and styrene derivatives (such as a-methylstyrene), as
long as the non-cyanoacrylate comonomers do not inhibit the ablation
process. The monomers used to provide these polymers can be alkyl
cyanoacrylates, alkoxy cyanoacrylates, and alkoxyalkyl cyanoacrylates.
Representative examples of poly(cyanoacrylates) include but are not
limited to poly(alkyl cyanoacrylates) and poly(alkoxyalkyl
cyanoacrylates) such as poly(methyl-2-cyanoacrylate),
poly(ethyl-2-cyanoacrylate), poly(methoxyethyl-2-cyanoacrylate),
poly(ethoxyethyl-2-cyanoacylate),
poly(methyl-2-cyanoacrylate-co-ethyl-2-cyanoacrylate), and other polymers
described in U.S. Pat. No. 5,998,088 (Robello et al.)

[0068] In still other embodiments, the laser-engraveable elastomeric
composition can include an alkyl-substituted polycarbonate or
polycarbonate block copolymer that forms a cyclic alkylene carbonate as
the predominant low molecular weight product during depolymerization from
engraving. The polycarbonate can be amorphous or crystalline, and can be
obtained from a number of commercial sources including Aldrich Chemical
Company (Milwaukee, Wis.). Representative polycarbonates are described
for example in U.S. Pat. No. 5,156,938 (Foley et al.), columns 9-12,
which are incorporated herein by reference. These polymers can be
obtained from various commercial sources or prepared using known
synthetic methods.

[0069] In still other embodiments, the laser-engraveable polymeric binder
is a polycarbonate (tBOC type) that forms a diol and diene as the
predominant low molecular weight products from depolymerization during
laser-engraving.

[0070] The laser-engraveable elastomeric composition generally comprises
at least 10 weight % and up to 99 weight %, and typically from about 30
to about 80 weight %, of the laser-engraveable elastomers or vulcanized
rubbers.

[0071] In some embodiments, inert microcapsules are dispersed within
laser-engraveable polymeric binders. For example, microcapsules can be
dispersed within polymers or polymeric binders, or within the crosslinked
elastomers or rubbery resins. The "microcapsules" can also be known as
"hollow beads", "microspheres", microbubbles", "micro-balloons", "porous
beads", or "porous particles". Such components generally include a
thermoplastic polymeric outer shell and either core of air or a volatile
liquid such as isopentane and isobutane. These microcapsules can include
a single center core or many interconnected or non-connected voids within
the core. For example, microcapsules can be designed like those described
in U.S. Pat. No. 4,060,032 (Evans) and U.S. Pat. No. 6,989,220 (Kanga),
or as plastic micro-balloons as described for example in U.S. Pat. No.
6,090,529 (Gelbart) and U.S. Pat. No. 6,159,659 (Gelbart).

[0072] The laser-engraveable, relief-forming layer composed of the
elastomeric composition can also include one or more infrared radiation
absorbing compounds that absorb IR radiation in the range of from about
750 to about 1400 nm or typically from 750 to 1250 nm, and transfer the
exposing photons into thermal energy. Particularly useful infrared
radiation absorbing compounds are responsive to exposure from IR lasers.
Mixtures of the same or different type of infrared radiation absorbing
compound can be used if desired. A wide range of infrared radiation
absorbing compounds are useful in the present invention, including carbon
blacks and other IR-absorbing organic or inorganic pigments (including
squarylium, cyanine, merocyanine, indolizine, pyrylium, metal
phthalocyanines, and metal dithiolene pigments), iron oxides and other
metal oxides.

[0073] Additional useful IR radiation absorbing compounds include carbon
blacks that are surface-functionalized with solubilizing groups are well
known in the art. Carbon blacks that are grafted to hydrophilic, nonionic
polymers, such as FX-GE-003 (manufactured by Nippon Shokubai), or which
are surface-functionalized with anionic groups, such as CAB-O-JET®
200 or CAB-O-JET® 300 (manufactured by the Cabot Corporation) are
also useful. Other useful pigments include, but are not limited to,
Heliogen Green, Nigrosine Base, iron (III) oxides, transparent iron
oxides, magnetic pigments, manganese oxide, Prussian Blue, and Paris
Blue. Other useful IR radiation absorbing compounds are carbon nanotubes,
such as single- and multi-walled carbon nanotubes, graphite, graphene,
and porous graphite.

[0075] Optional addenda in the laser-engraveable elastomeric composition
can include but are not limited to, plasticizers, dyes, fillers,
antioxidants, antiozonants, stabilizers, dispersing aids, surfactants,
dyes or colorants for color control, and adhesion promoters, as long as
they do not interfere with engraving efficiency.

[0076] The flexographic printing precursor can be formed from a
formulation comprising a coating solvent, one or more elastomeric resins,
and an infrared radiation absorbing compound, to provide an elastomeric
composition. This formulation can be formed as a self-supporting layer or
applied to a suitable substrate. Such layers can be formed in any
suitable fashion, for example by injecting, spraying, or pouring a series
of formulations to the substrate. Alternatively, the formulations can be
press-molded, injection-molded, melt extruded, co-extruded, or melt
calendared into an appropriate layer or ring (sleeve) and optionally
adhered or laminated to a substrate and cured to form a layer, flat or
curved sheet, or seamless printing sleeve. The flexographic printing
precursors in sheet-form can be wrapped around a printing cylinder and
fused at the edges to form a seamless printing precursor.

Method of Forming Flexographic Printing Member

[0077] Ablation or engraving energy can be applied using a suitable laser
such as a CO2, infrared radiation-emitting diode, or YAG lasers, or
an array of such lasers. Ablation engraving is used to provide a relief
image with a minimum floor depth of at least 100 μm or typically from
300 to 1000 μm. However, local minimum depths between halftone dots
can be less. The relief image may have a maximum depth up to about 100%
of the original thickness of the laser-engraveable, relief-forming layer
when a substrate is present. In such instances, the floor of the relief
image can be the substrate if the laser-engraveable, relief-forming layer
is completely removed in the image area, a lower region of the
laser-engraveable, relief-forming layer, or an underlayer such as an
adhesive layer, compliant layer, or a non-ablative elastomeric or rubber
underlayer. When a substrate is absent, the relief image can have a
maximum depth of up to 80% of the original thickness of the
laser-engraveable, relief-forming layer comprising the elastomeric
composition. A laser operating at a wavelength of from about 700 nm to
about 11 μm is generally used, and a laser operating at from 800 nm to
1250 nm is more preferable. The laser must have a high enough intensity
that the pulse or the effective pulse caused by relative movement is
deposited approximately adiabatically during the pulse. Pulse duration is
typically much less than 1 ms.

[0078] Generally, engraving is achieved using at least one infrared
radiation laser having a minimum fluence level of at least 1 J/cm2
at the elastomeric topmost surface and typically infrared imaging is at
from about 20 to about 1000 J/cm2 or more preferably from about 50
to about 800 J/cm2.

[0079] Engraving a relief image can occur in various contexts. For
example, sheet-like precursors can be imaged and used as desired, or
wrapped around a printing cylinder or cylinder form before imaging. The
flexographic printing precursor can also be a printing sleeve that can be
imaged before or after mounting on a printing cylinder.

[0080] During imaging, most of the removed products of engraving are
gaseous or volatile and readily collected by vacuum for disposal or
chemical treatment. Any solid debris can be similarly collected using
vacuum or washing.

[0081] After imaging, the resulting flexographic printing member can be
subjected to an optional detacking step if the elastomeric topmost
surface is still tacky, using methods known in the art.

[0082] During printing, the resulting flexographic printing member is
inked using known methods and the ink is appropriately transferred to a
suitable receiver element.

[0083] After printing, the flexographic printing member can be cleaned and
reused. The printing cylinder can be scraped or otherwise cleaned and
reused as needed.

[0084] Referring now to FIG. 1a shows a comparative flexographic member
60, for example, plate or sleeve, having an original top most
flexographic plate member surface 30 and floor relief level 20 with an
engraved relief pattern having coarse features, 50, 51 and coarse
highlight features 40, 41, 42, 43, floor region extent 10 and a
stress-sensitive boundary 11. The top most surface of coarse highlight
features has dot size 48. The side walls of features in this diagram are
represented as straight sloped walls and have a lowest inter-dot relief
at level 8 but it is understood that the side walls of the actual relief
image can be vertical, sloped or curved or can have plateaus below the
top most surface of the feature or any combination of these patterns. In
reference to the field of highlight features, the outer-dot coarse
highlight feature 40 represents the dots closest to the stress-sensitive
boundary and inner-dot coarse highlight feature 43 represents dots
farthest from stress-sensitive boundaries.

[0085] The current invention can be understood with reference to a
cross-sectional diagram of the current invention in FIG. 1b showing laser
radiation, 100, used to selectively modify the floor by engraving
floor-interface stress-sensitive boundary increased relief 14 having a
floor-interface stress-sensitive boundary increased relief width 16 at a
floor-interface stress-sensitive boundary 11. The coarse features have an
original top most surface coincident with the top most surface 30 of the
flexographic member 60 prior to engraving. Laser radiation engraves down
to a floor-interface stress-sensitive boundary relief level 18 at least
30 μm below the floor level 20 in the final relief image. In this
example the stress-sensitive boundary 11 occurs at the boundary between
the highlight features 40, 41, 42, 43, and the floor region 10. A cut of
increased relief 14 has a depth of at least 30 μm below the floor
level 20 and a width 16 of at least 10 μm and less than half 19 the
total distance to the next closest printable feature.

[0086] FIG. 2a shows a comparative flexographic member 60, having an
original top most flexographic plate member surface 30 and floor level 20
with an engraved relief pattern having coarse features 50, 51 and
highlight features 40, 41, 42, 43, floor region 10, and boundary 11. The
side walls at the boundary 11 of this representative illustration have a
non-printable plateau 12 and a non-printable plateau angled wall 24.

[0087] An embodiment of the current invention is schematically represented
in FIG. 2b showing laser radiation 100 used to selectively engrave cuts
of increased relief 14 having a width 16 at a stress-sensitive boundary
11. Laser radiation 100 engraves down to a level 18 at least 10 μm
below the floor relief level 20. The width 16 of the cut is at least 10
μm and less than half 19 the total distance to the next printable
feature.

[0088] FIG. 3a is a schematic cross-sectional diagram illustrating a
comparative flexographic member having coarse features 50, and fine
features 70, 71, 75, 77 having an inter dot spacing 27 determined by the
halftone screen ruling and fine-feature top most level 6, below the
critical printable level 9 for transferring ink on press. The
flexographic member is shown with a supporting layer 80 and a backing
layer 90. The fine features 70, 71, 75, 77 have coarse highlight feature
dot size 48 that are small compared to size of the spot used to laser
engrave the relief pattern and are therefore "naturally" undercut to a
level 6 below a critical level 9 that results in features that print
chaotically or not at all on press. Fine coarse interface
stress-sensitive boundary 13 are illustrated between the last rows of
fine-feature dots and the coarse features. Fine feature inner dot 77
represents the dot farthest from the stress-sensitive boundary and fine
feature outer dots 70 represent the dots closest to the stress-sensitive
boundary.

[0089] A representation of another embodiment of the current invention,
FIG. 3b, shows laser radiation, 100, used to selectively engrave cuts of
fine coarse interface stress-sensitive boundary increased relief 15
having a fine coarse interface stress-sensitive boundary width 17 at a
boundary 13 between the last rows of fine-feature dots 70, 71, 75, 77 and
the coarse features 50. The cuts are engraved down to a fine coarse
interface stress-sensitive boundary relief level 26 at least 30 μm
below the lowest inter-dot relief level 8. The width 17 is at least 10
μm and no wider than the inter-dot spacing 27 determined by the screen
ruling of the halftone image.

[0090] FIG. 4 shows an apparatus for preparing a flexographic printing
plate according to the present invention. A flexographic printing member
60 is mounted on a drum 110 which is turned by motor 130. A lead screw
150 is driven by a lead screw motor 155. A printhead platform 190 is
attached to lead screw 150 which moves the platform parallel to a surface
of the drum. A laser thermal printhead 170 is mounted on the platform for
imaging the flexographic printing member. A lens 175 directs laser
radiation 100 to the flexographic printing member. Electrical leads 140
connect various pieces of the apparatus with computer 160 coordinating
movement of the drum 110, lead screw 150, and operation of the laser
thermal printhead 170. A debris collection system 180 collects detritus
generated by laser thermal engraving. A relief image with coarse and fine
features is created as described above.

Modeling

[0091] A linear elastic two-dimensional model was used to simulate the
press interactions between a flexographic member and a receiver and to
make predictions for the dot gain and dot gain uniformity associated with
the increase in lateral dimension of the top most surface of features
under engagement. All materials were treated as isotropic. The model did
not contain corrections for non-linearity, known to be important for real
elastomers, but the linear model is expected to qualitatively capture the
correct trends for phenomenon of interest here provided that the strains
are not too high. Also, the model did not specifically include effects
due to ink flow and therefore did not include that contribution to dot
gain nor the halo effect observed in actual prints. Again, it is expected
that the predicted variation in stress at different positions in the
flexographic member would lead to qualitatively similar trends for ink
spread. Results were found to be in good qualitative agreement with
printed examples. Two types of stress-sensitive boundaries, as described
above, were investigated and improvements provided by cuts were
demonstrated.

[0092] The following descriptions of the model predictions can be
understood with reference to the tables and figures herein. All distances
are in microns unless specified otherwise. One problem to be solved is
characterized in Table 1 and FIG. 1a where a floor-interface
stress-sensitive boundary, 11, between a course feature region 40, 41,
42, and 43 and floor region was modeled as a function of floor extent 10.
In this case a row of four dots, initially 50 μm across at their top
most surfaces dot size 48 were near a floor region having 400 μm of
relief. In Table 1, the dot 40, closest to the stress-sensitive boundary
is referred to as the outer dot and the one furthest from the boundary 43
as the inner dot. A next-nearest-neighbor coarse feature 51, was (100
μm) at its top most surface, and was separated from the
stress-sensitive boundary by the floor region with an extent 10 specified
in Table 1. Infinity here refers to the limiting case without a
next-nearest-neighbor coarse feature 51 (i.e. infinitely far away). The
model included one supporting layer representing, for example, a
polyester support and another subbing layer representing, for example, a
backing tape. The top most surface of the 1.7 mm thick flexographic
member 60 was subjected to a strain equivalent to 200 microns of
engagement. The computed dot size under stress of the inner and outer
dots and a measure of dot gain non-uniformity, NU given by

% NU=100 (Outer size-Inner size)/Inner size (1)

are reported in Table 1. Results showed 4% non-uniformity when no solid
feature was present improving to 2% when the nearest neighbor coarse
feature was about 1000 μm away. As the extent of the floor region was
reduced the non-uniformity monotonically improved. At 225 μm the
non-uniformity was less than half a percent.

[0093] Table 2 shows the improvement achievable when a cut of 400 μm of
increased relief, 50 μm wide, was engraved at the stress-sensitive
boundary. In this case the original dot size 48 before engagement was 25
μm and no nearest neighbor solid was present (i.e. infinite floor
extent). The calculated non-uniformity was improved from 4.9% to 1.6%,
better than a factor of 3 when the cut was made at the stress-sensitive
boundary.

[0094] Predicted improvements achievable when the stress-sensitive
boundary contains a non-printable plateau 12 (see FIG. 2a) are reported
in Table 3. In this case initial dot size 48 was 25 μm, the floor
relief was 500 μm and the plateau 12 was 50 μm wide. The cuts, as
illustrated in FIG. 2b, were 25 μm wide and had depths as indicated in
the Table 3. The calculated non-uniformity improved from 6.7% without
cuts to 0.3% with 1550 μm cuts.

[0095] Table 4 compares the predictions for 1.2 mm and 1.7 mm thick
flexographic members as a function of dot size and cut depth. It was
found that non-uniformity was worse for smaller dots as has been observed
in real press runs and that non-uniformity was generally worse for
thinner flexographic members. Modeling showed that cuts at the
stress-sensitive boundary improved non-uniformity and that deeper cuts
were better.

[0096] The problem and solution of the current invention for
fine-coarse-interface stress-sensitive boundaries are described with
reference to Table 5 and FIGS. 3a and 3b. The displacement between the
initial levels 6 of the fine feature inner dot 77 and fine feature outer
dots 70 before and after 150 μm of engagement on the coarse features
50 are shown Table 5. The engagement non-uniformity, ENU, as a function
of cut depth 26 given by

ENU=Inner displacement-Outer displacement (2)

is also reported in the table. The displacements and ENUs were calculated
for combinations of subbing layer hardness. The first descriptor in
column 1 Table 5 refers to the relative hardness of the supporting layer
80 and the second to the backing layer 90. The Young's modulus of the
flexographic member 60 was 1 (dimensionless units), the soft layers were
0.1 and the hard layers were 10. Cut width 17 was 20 μm and depth 26
was as indicated in Table 5. Results show monotonic improvement of
engagement non-uniformity in all cases with increasing cut depth. The
cuts were most effective for hard supporting and subbing layers.

[0097] The invention has been described in detail with particular
reference to certain preferred embodiments thereof, but it will be
understood that variations and modifications can be effected within the
spirit and scope of the invention.